1. Field of the Invention
The invention relates to a fuel cell.
2. Description of Related Art
There is a known technique in which a fuel cell is operated at a low-efficiency operating point when a warm-up operation of the fuel cell is required (refer to Japanese Patent Application Publication No. 2007-184243 (JP 2007-184243 A)). According to the technique described in JP 2007-184243 A, in order to operate the fuel cell at a low-efficiency operating point, the generated voltage is decreased with the use of a converter, and the feed rate of cathode gas fed by an air compressor is reduced. With this control, the operating point is changed with the output kept constant, so that the fuel cell shifts to a warm-up operation.
With regard to the above-described fuel cell, sufficient consideration is not given to stable continuation of the warm-up operation. For example, when the output is changed during continuation of the warm-up operation, the converter and the air compressor are controlled to change the operating point of the fuel cell. For example, in the case where the converter is configured to be controlled such that the generated current approaches a target value, the target value of the generated current is increased when the output is increased, and the target value of the generated current is decreased when the output is decreased. In parallel with the control of the converter, the feed rate of the cathode gas fed by the air compressor is regulated such that a targeted output is achieved. It is known that, in a fuel cell, the generated current and the generated voltage exhibit a specific correlation when the amounts of hydrogen and oxygen to be fed are constant. Hereinafter, the correlation will be referred to as “I-V characteristic”.
The control of the feed rate of the cathode gas fed by the air compressor is lower in responsiveness than the control by the converter. Thus, in some cases, the operating point is changed in a situation where the control of the generated current by the converter is dominant control.
It should be noted that, depending on the operating point of the fuel cell, a decrease in the generated current may cause a rise in the output rather (the detail will be described with reference to FIG. 4). Therefore, when the output is decreased only by a decrease in the generated current in order to decrease the output from such an operating point, the operating point is changed to an operating point at which the generated current is lower than the generated current at an operating point at which the output reaches the peak. Generally, the I-V characteristic exhibits the following tendency: in a region in which the generated current is low, the lower the generated current is, the higher the generated voltage is. Therefore, when the operating point is changed in the above-described manner, the generated voltage becomes higher. On the other hand, as described later in detail, when the generated voltage is high, the amount of heat generation is small. Therefore, when the operating point is changed in the above-described manner, the amount of heat generation becomes small and thus the warm-up operation is interrupted.